Nonaqueous electrolytic solution and lithium ion secondary battery

ABSTRACT

A nonaqueous electrolytic solution comprising a nonaqueous solvent, an electrolyte salt comprising a lithium salt, and a difluoroboron complex compound represented by the following general formula (1): 
                         
wherein R 1  and R 2  each independently represent a substituted or unsubstituted alkyl group having 1-6 carbon atoms, a substituted or unsubstituted aryl group, a substituted or unsubstituted heteroaryl group, or a substituted or unsubstituted alkoxy group, and R 3  represents a hydrogen atom, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heteroaryl group.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application of InternationalApplication No. PCT/JP2015/069033 entitled “NONAQUEOUS ELECTROLYTICSOLUTION AND LITHIUM ION SECONDARY BATTERY,” filed on Jul. 1, 2015,which claims the benefit of the priority of Japanese patent applicationNo. 2014-152070, filed on Jul. 25, 2014, the disclosures of each ofwhich are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a nonaqueous electrolytic solution anda lithium ion secondary battery.

BACKGROUND ART

Nonaqueous electrolyte secondary batteries such as lithium ion secondarybatteries have already been put into practical use as a battery forsmall electronic devices such as notebook computers and cellular phonesbecause of their advantages such as high energy density, smallself-discharge, and excellent long-term reliability. In recent years,nonaqueous electrolyte secondary batteries have been more and moreutilized for a battery for electrical vehicles, a battery for householduse, and a battery for power storage.

A lithium ion secondary battery includes a positive electrode primarilycomprising a positive electrode active material and a negative electrodecontaining a material capable of intercalating and deintercalating alithium ion as a main component, and a nonaqueous electrolytic solution.Examples of positive electrode active materials used for a positiveelectrode include lithium metal oxides such as LiCoO₂, LiMnO₂, LiNiO₂,LiFePO₄, and LiMn₂O₄.

Examples of negative electrode active materials used for a negativeelectrode include: metal lithium; and silicon, oxides such as siliconoxides, and carbonaceous materials each of which is capable ofintercalating and deintercalating a lithium ion. In particular, lithiumion secondary batteries with a carbonaceous material capable ofintercalating and deintercalating a lithium ion such as graphite(artificial graphite, natural graphite) and coke have already been putin practical use.

Examples of nonaqueous electrolytic solutions used include a solutionobtained by adding a lithium salt such as LiPF₆, LiBF₄, LiN(SO₂CF₃)₂,LiN(SO₂C₂F₅)₂, and lithium bis(oxalate)borate(LiB(C₂O₄)₂) to a mixedsolvent of a cyclic carbonate solvent such as ethylene carbonate andpropylene carbonate, and a linear carbonate solvent such as dimethylcarbonate, diethyl carbonate, and ethyl methyl carbonate.

In a secondary battery using such a nonaqueous electrolytic solution,for example, a solvent in the electrolytic solution is reduced anddecomposed on the surface of a negative electrode, especially under ahigh temperature environment, and the decomposition product deposits onthe surface of the negative electrode to increase the resistance, or agas generated through the decomposition of the solvent causes thebattery to swell. On the surface of a positive electrode, the solvent isoxidized and decomposed, and the decomposition product deposits on thesurface of the positive electrode to increase the resistance, or a gasgenerated through the decomposition of the solvent causes the battery toswell. As a result, the storage characteristics of a battery under ahigh temperature environment and the cycle characteristics of asecondary battery are lowered, which disadvantageously causesdegradation of battery characteristics.

To prevent the occurrence of such problems, a compound having a functionto form a protective film is added into a nonaqueous electrolyticsolution. Specifically, it is known that the compound added into anelectrolytic solution is intentionally allowed to decompose on thesurface of an electrode active material in initial charging so that thedecomposition product forms a protective film having a protectivefunction to prevent further decomposition of a solvent, or an SEI (SolidElectrolyte Interface). It has been reported that the protective filmformed on the surface of an electrode suitably suppresses the chemicalreaction or decomposition of a solvent on the surface of an electrode,and as a result exerts an effect of maintaining the batterycharacteristics of a secondary battery (Non Patent Literature 1).Addition of, for example, vinylene carbonate, fluoroethylene carbonate,or maleic anhydride as an additive for formation of such a protectivefilm to an electrolytic solution has been attempted to improve batterycharacteristics (Non Patent Literature 1).

On the other hand, addition of a boron compound to an electrolyticsolution to reduce capacity degradation due to repeatedcharging/discharging is known.

For example, Patent Literature 1 describes use of a fluorinated boroncompound selected from the group consisting of BF₃, a BF₃ complex, HBF₄,and an HBF₄ complex as an additive for an electrolyte in a nonaqueouslithium battery. In addition, the patent literature describesBF₃-diethyl carbonate complex, BF₃-ethyl methyl carbonate complex, andthe like as the BF₃ complex, and describes BF₄-diethyl carbonate complexas the HBF₄ complex.

Patent Literature 2 describes blending a boron compound in anelectrolytic solution with a first lithium salt dissolved in anonaqueous solvent, together with a lithium salt of an organic acid as asecond lithium salt. The patent literature describes boron trifluorideand halogenated boron complexes as the boron compound, and describesboron trifluoride-diethyl ether complex, boron trifluoride-di-n-butylether complex, and boron trifluoride-tetrahydrofuran complex as thehalogenated boron complex.

CITATION LIST Patent Literature

-   Patent Literature 1:

JP11-149943A

-   Patent Literature 2:

JP2012-248519A

-   Non Patent Literature-   Non Patent Literature 1:

Journal. Power Sources, vol. 162, p. 1379-1394 (2006)

SUMMARY OF INVENTION Technical Problem

Even if a nonaqueous electrolytic solution comprising an additivedescribed in Non Patent Literature 1 or Patent Literature 1 or 2 isused, however, reduction of degradation of battery characteristics undera high temperature environment is insufficient, and a nonaqueouselectrolytic solution comprising an additive to provide a furtherimprovement effect is required.

The present invention was made in view of the above problem, and anobject of the present invention is to provide a nonaqueous electrolyticsolution capable of reducing degradation of battery characteristicsunder a high temperature environment, and a lithium ion secondarybattery using the nonaqueous electrolytic solution which has excellentbattery characteristics.

Solution to Problem

A nonaqueous electrolytic solution according to one aspect of thepresent invention comprises a nonaqueous solvent, an electrolyte saltcomprising a lithium salt, and a difluoroboron complex compoundrepresented by the following general formula (1).

In the formula, R¹ and R² each independently represent a substituted orunsubstituted alkyl group having 1-6 carbon atoms, a substituted orunsubstituted aryl group, a substituted or unsubstituted heteroarylgroup, or a substituted or unsubstituted alkoxy group, and R³ representsa hydrogen atom, a substituted or unsubstituted alkyl group having 1-6carbon atoms, a substituted or unsubstituted aryl group, or asubstituted or unsubstituted heteroaryl group.

A lithium ion secondary battery according to another aspect of thepresent invention includes: a positive electrode comprising a positiveelectrode active material capable of intercalating and deintercalating alithium ion; a negative electrode comprising a negative electrode activematerial capable of intercalating and deintercalating a lithium ion; andthe above nonaqueous electrolytic solution.

Advantageous Effects of Invention

Exemplary embodiments can provide a nonaqueous electrolytic solutioncapable of reducing degradation of battery characteristics under a hightemperature environment, and a lithium ion secondary battery havingexcellent battery characteristics.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a schematic cross-sectional view to illustrate theconfiguration of a lithium ion secondary battery according to anexemplary embodiment.

DESCRIPTION OF EMBODIMENTS

The present inventors found as a result of diligent study to solve theabove problem that addition of a difluoroboron complex compound having aspecific structure to a nonaqueous electrolytic solution can improvebattery characteristics such as cycle characteristics under a hightemperature environment, and thus completed the present invention.

Specifically, a nonaqueous electrolytic solution according to anexemplary embodiment comprises a nonaqueous solvent, a lithium salt asan electrolyte salt, and a difluoroboron complex compound represented bythe above general formula (1).

The nonaqueous electrolytic solution may comprise one or two or more ofthe difluoroboron complex compounds. The amount of the difluoroboroncomplex compound to be added (content) is preferably within the range of0.01 to 10% by mass based on the total mass of the nonaqueouselectrolytic solution.

The nonaqueous electrolytic solution according to an exemplaryembodiment may further comprise at least one additive compound selectedfrom the group consisting of vinylene carbonate, fluoroethylenecarbonate, 1,3-propanesultone, maleic anhydride, and1,5,2,4-dioxadithiane-2,2,4,4-tetraoxide. The amount of the additivecompound to be added (content) is preferably within the range of 0.01 to10% by mass based on the total mass of the nonaqueous electrolyticsolution.

The nonaqueous electrolytic solution according to an exemplaryembodiment preferably comprises a carbonate as the nonaqueous solvent,and more preferably comprises a cyclic carbonate and a linear carbonate.

The concentration of the electrolyte salt in the nonaqueous electrolyticsolution according to an exemplary embodiment is preferably within therange of 0.1 to 3 mol/L.

A lithium ion secondary battery according to another exemplaryembodiment includes: a positive electrode comprising a positiveelectrode active material capable of intercalating and deintercalating alithium ion; a negative electrode comprising a negative electrode activematerial capable of intercalating and deintercalating a lithium ion; andthe above nonaqueous electrolytic solution. The negative electrodeactive material preferably comprises at least one selected from thegroup consisting of elementary silicon, a silicon oxide, and acarbonaceous material.

Presumably, the difluoroboron complex compound represented by thegeneral formula (1) undergoes chemical reaction on the surface of anelectrode active material in initial charging of a battery, and theproduct forms a protective film having a protective function to preventfurther decomposition of an electrolytic solution, or an SEI (SolidElectrolyte Interface), on the surface of an electrode. Thedifluoroboron complex compound represented by the general formula (1)added forms a protective film on the surface of an electrode to suitablysuppress the chemical reaction or decomposition of an electrolyticsolution on the surface of an electrode, and as a result an effect ofmaintaining the long-term reliability and lifetime of a secondarybattery is provided. By virtue of this, a secondary battery can beprovided which has a large capacity and high energy density and isexcellent in stability in charge/discharge cycles, and in whichdegradation of battery characteristics is reduced even under a hightemperature environment.

Now, the nonaqueous electrolytic solution according to an exemplaryembodiment and a lithium ion secondary battery using it will bedescribed in detail.

[Additive Component of Nonaqueous Electrolytic Solution]

The nonaqueous electrolytic solution according to an exemplaryembodiment comprises at least one difluoroboron complex compoundrepresented by the following general formula (1).

In the formula (1), R¹ and R² each independently represent a substitutedor unsubstituted alkyl group having 1-6 carbon atoms, a substituted orunsubstituted aryl group, a substituted or unsubstituted heteroarylgroup, or a substituted or unsubstituted alkoxy group, and R³ representsa hydrogen atom, a substituted or unsubstituted alkyl group having 1-6carbon atoms, a substituted or unsubstituted aryl group, or asubstituted or unsubstituted heteroaryl group.

In the case that R¹ and R² are different, the difluoroboron complexaccording to an exemplary embodiment is expected to have isomersrepresented by formula (1A) and formula (1B). In the presentspecification, even in the case that only the structural formula (1A) isshown as the structure of the difluoroboron complex, the structuralformula (1B) as an isomer is also included therein unless otherwisestated.

Examples of the substituted or unsubstituted alkyl group having 1-6carbon atoms for R¹, R², and R³ include unsubstituted alkyl groups suchas a methyl group, an ethyl group, a propyl group, an isopropyl group, an-butyl group, an isobutyl group, a t-butyl group, a pentyl group, and an-hexyl, and alkyl groups with one or more hydrogen atoms of the alkylgroup replaced with a substituent. Examples of the substituent include afluorine atom, a cyano group, an ester group having 1-5 carbon atoms(—COOZ, where Z is an alkyl group), an alkoxy group having 1-5 carbonatoms, an aryl group, and a heteroaryl group (thienyl group, furanylgroup, etc.). Two or more hydrogen atoms of the alkyl group may be eachindependently replaced with a different substituent. Examples of thesubstituted alkyl group include a trifluoromethyl group, apentafluoroethyl group, a trifluoroethyl group, a heptafluoropropylgroup, a cyanomethyl group, a benzyl group, and a 2-thienylmethyl group.

Examples of the substituted or unsubstituted aryl group for R¹, R², andR³ include unsubstituted aryl groups such as a phenyl group and anaphthyl group, and aryl groups with one or more hydrogen atoms of thearyl group replaced with a substituent. Examples of the substituentinclude an alkyl group having 1-5 carbon atoms, a fluorine atom, a cyanogroup, and an alkoxy group having 1-5 carbon atoms. Two or more hydrogenatoms of the aryl group may be each independently replaced with adifferent substituent. Examples of the substituted aryl group include atolyl group, a 4-cyanophenyl group, a 2-fluorophenyl group, a3-fluorophenyl group, a 4-fluorophenyl group, a 2,3-difluorophenylgroup, a 2,4-difluorophenyl group, a 2,5-difluorophenyl group, a2,6-difluorophenyl group, a 3,4-difluorophenyl group, a3,5-difluorophenyl group, a 3,6-difluorophenyl group, a2,4,6-trifluorophenyl group, a pentafluorophenyl group, and a4-methoxyphenyl group.

Examples of the substituted or unsubstituted heteroaryl group for R¹,R², and R³ include unsubstituted heteroaryl groups such as a thienylgroup (2-thienyl group, 3-thienyl group) and a furanyl group (e.g.,2-furanyl group), and heteroaryl groups with one or more hydrogen atomsof the heteroaryl group replaced with a substituent. Examples of thesubstituent include an alkyl group having 1-5 carbon atoms, a fluorineatom, a cyano group, and an alkoxy group having 1-5 carbon atoms. Two ormore hydrogen atoms of the heteroaryl group may be each independentlyreplaced with a different substituent. Examples of the substitutedheteroaryl group include a 4-methyl-2-thienyl group and a3-fluoro-2-thienyl group.

Examples of the substituted or unsubstituted alkoxy group for R¹ and R²include unsubstituted alkoxy groups having 1-5 carbon atoms such as amethoxy group, an ethoxy group, a propoxy group, and a butoxy group, andsubstituted alkoxy groups obtained by substituting one or more hydrogenatoms of an alkoxy group having 1-5 carbon atoms with a substituent suchas a benzyloxy group. Examples of the substituent include a fluorineatom, a cyano group, an aryl group, and a heteroaryl group (thienylgroup, furanyl group, etc.).

In a preferred example, R¹ and R² are each independently a methyl group,a trifluoromethyl group, a pentafluoroethyl group, a phenyl group, a2-thienyl group, a 2-furanyl group, a 2-fluorophenyl group, apentafluorophenyl group, a 4-fluorophenyl group, a 2,4-difluorophenylgroup, a 4-cyanophenyl group, an ethoxy group, or a methoxy group.

In a preferred example, R³ is a hydrogen atom, a phenyl group, a2-thienyl group, a 4-fluorophenyl group, a 2,4-difluorophenyl group, ora pentafluorophenyl group.

Specific examples of the compound represented by the above generalformula (1) are shown in Table 1; however, the present invention isnever limited thereto.

TABLE 1 Compound Structural formula FB1

FB2

FB3

FB4

FB5

FB6

FB7

FB8

FB9

FB10

FB11

FB12

FB13

The difluoroboron complex compound represented by the general formula(1) can be obtained, for example, by using a production method describedin Tetrahedron, vol. 63, p. 9357-9358 (2007).

Examples of production methods for the difluoroboron complex compoundrepresented by the general formula (1) include a method of reacting adiketone represented by the following formula (A-a) and borontrifluoride-diethyl ether complex with use of an appropriate solvent.

In the formula, R¹, R², and R³ are the same as R¹, R², and R³ in thegeneral formula (1).

Examples of the solvent which can be used in the production methodinclude: halogenated hydrocarbons such as methylene chloride,1,2-dichloroethane, and chloroform; 1,2-dimethoxyethane; andacetonitrile. Among them, methylene chloride, 1,2-dichloroethane, and1,2-dimethoxyethane, etc., are preferred.

To the nonaqueous electrolytic solution according to an exemplaryembodiment, the difluoroboron complex compound represented by the abovegeneral formula (1) is added. The content (amount added) of thedifluoroboron complex compound in the nonaqueous electrolytic solutionis preferably within the range of 0.01 to 10% by mass, more preferablywithin the range of 0.02 to 5% by mass, and even more preferably withinthe range of 0.03 to 3% by mass based on the total mass of thenonaqueous electrolytic solution. If the content (amount added) of thedifluoroboron complex compound is 0.01% by mass or more, a sufficienteffect of addition can be achieved. If the content (amount added) of thedifluoroboron complex compound is 10% by mass or less, the cost can bereduced concomitantly with achievement of a sufficient effect ofaddition.

To the nonaqueous electrolytic solution according to an exemplaryembodiment, only one of the difluoroboron complex compounds representedby the above general formula (1) may be added, or two or more thereofmay be added.

In addition to the difluoroboron complex compound represented by thegeneral formula (1), the nonaqueous electrolytic solution according toan exemplary embodiment may optionally comprise a known additivecompound for nonaqueous electrolytic solutions as an additional additivecomponent. Examples of the additional additive component includevinylene carbonate, fluoroethylene carbonate, maleic anhydride, ethylenesulfite, boronates, 1,3-propanesultone, and1,5,2,4-dioxadithiane-2,2,4,4-tetraoxide. Among them, vinylenecarbonate, fluoroethylene carbonate, 1,3-propanesultone, maleicanhydride, and 1,5,2,4-dioxadithiane-2,2,4,4-tetraoxide are preferred.One of these additional additive compounds may be used singly, or two ormore thereof may be used in combination.

[Nonaqueous Solvent]

The nonaqueous solvent (nonaqueous organic solvent) used in thenonaqueous electrolytic solution according to an exemplary embodiment isnot limited, and can be appropriately selected from common nonaqueoussolvents. For example, a nonaqueous solvent comprising at least onesolvent selected from the group consisting of a cyclic carbonate, alinear carbonate, a linear ester, a lactone, an ether, a sulfone, anitrile, and a phosphate can be used.

Specific examples of the cyclic carbonate include propylene carbonate,ethylene carbonate, fluoroethylene carbonate, butylene carbonate,vinylene carbonate, and vinylethylene carbonate.

Specific examples of the linear carbonate include dimethyl carbonate,diethyl carbonate, dipropyl carbonate, dibutyl carbonate, ethyl methylcarbonate, methyl propyl carbonate, methyl isopropyl carbonate, andmethyl butyl carbonate.

Specific examples of the linear ester include carboxylates such asmethyl formate, methyl acetate, methyl propionate, ethyl propionate,methyl pivalate, and ethyl pivalate.

Specific examples of the lactone include γ-butyrolactone,δ-valerolactone, and α-methyl-γ-butyrolactone.

Specific examples of the ether include tetrahydrofuran,2-methyltetrahydrofuran, 1,3-dioxolane, 1,3-dioxane, 1,4-dioxane,1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxy ethane.

Specific examples of the sulfone include sulfolane, 3-methylsulfolane,and 2,4-dimethylsulfolane.

Specific examples of the nitrile include acetonitrile, propionitrile,succinonitrile, glutaronitrile, and adiponitrile.

Specific examples of the phosphate include trimethyl phosphate, triethylphosphate, tributyl phosphate, and trioctyl phosphate.

One of the above nonaqueous solvents may be used singly, or two or morethereof may be used in a mixture. Examples of the combination include acombination of a cyclic carbonate and a linear carbonate, and acombination of a cyclic carbonate and a linear carbonate with additionof a linear ester, a lactone, an ether, a nitrile, a sulfone, or aphosphate as a third solvent. Among them, combinations at leastcomprising a cyclic carbonate and a linear carbonate are preferred forachieving excellent battery characteristics.

The nonaqueous solvent preferably contains a cyclic carbonate. Sincecyclic carbonates have a large dielectric constant, addition of a cycliccarbonate can enhance the ion conductivity of the nonaqueouselectrolytic solution. The content of a cyclic carbonate contained inthe nonaqueous electrolytic solution is not limited, but preferably 5%by volume or more, more preferably 10% by volume or more, and even morepreferably 20% by volume and preferably 70% by volume or less, morepreferably 60% by volume or less, and even more preferably 50% by volumeor less, from the viewpoint of the ion conductivity, viscosity, etc., ofthe nonaqueous electrolytic solution.

[Electrolyte Salt]

Examples of the electrolyte salt contained in the nonaqueouselectrolytic solution according to an exemplary embodiment include, butnot limited to, lithium salts such as LiPF₆, LiBF₄, LiClO₄, LiN(SO₂F)₂,LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, CF₃SO₃Li, C₄F₉SO₃Li, LiAsF₆, LiAlCl₄,LiSbF₆, LiPF₄(CF₃)₂, LiPF₃(C₂F₅)₃, LiPF₃(CF₃)₃, (CF₂)₂(SO₂)₂NLi,(CF₂)₃(SO₂)₂Li, lithium bis(oxalate)borate, and lithiumoxalatodifluoroborate. Among them, LiPF₆, LiBF₄, LiN(SO₂F)₂,LiN(SO₂CF₃)₂, and LiN(SO₂C₂F₅)₂ are preferred. One of these electrolytesalts may be used singly, or two or more thereof may be used incombination.

The concentration of the electrolyte salt dissolved in the nonaqueoussolvent in the nonaqueous electrolytic solution is preferably within therange of 0.1 to 3 mol/L, and more preferably within the range of 0.5 to2 mol/L. If the concentration of the electrolyte salt is 0.1 mol/L ormore, a more sufficient ion conductivity can be achieved; and if theconcentration of the electrolyte salt is 3 mol/L or less, increase ofthe viscosity of the electrolytic solution can be reduced, and a moresufficient ion mobility and impregnating ability can be achieved.

[Lithium Ion Secondary Battery]

A lithium ion secondary battery according to an exemplary embodimentprimarily includes a positive electrode, a negative electrode, anonaqueous electrolytic solution (a nonaqueous electrolytic solutionwith the difluoroboron complex compound represented by the generalformula (1) and an electrolyte salt dissolved in a nonaqueous solvent),and a separator disposed between the positive electrode and the negativeelectrode. For the nonaqueous electrolytic solution, the above-describednonaqueous electrolytic solution can be suitably used. Constitutionalmembers other than the nonaqueous electrolytic solution such as thepositive electrode, the negative electrode, and the separator are notlimited, and common constitutional members for a typical lithium ionsecondary battery can be applied. Constitutional members other than thenonaqueous electrolytic solution suitable for the lithium ion secondarybattery according to an exemplary embodiment will be described below.

(Positive Electrode)

For the positive electrode in the lithium ion secondary batteryaccording to an exemplary embodiment, for example, a positive electrodein which a positive electrode active material layer comprising apositive electrode active material and a binder is formed to cover apositive electrode current collector can be used. The binder binds thepositive electrode active material and the current collector, and bindsthe positive electrode active material itself.

For the positive electrode active material, a lithium composite metaloxide comprising a transition metal such as cobalt, manganese, andnickel, and lithium can be used. Specific examples of the lithiumcomposite metal oxide include LiMnO₂, Li_(x)Mn₂O₄ (0<×<2), Li₂MnO₃-LiMO₂solid solutions (M=Co, Ni, etc.), LiCoO₂, LiNiO₂, LiCo_(1-x)Ni_(x)O₂(0.01<×<), LiNiv_(1/2)Mn_(3/2)O₄, and LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂. Inaddition, lithium composite metal oxides in which Li is present morethan the stoichiometric composition of the above lithium composite metaloxides are included.

To enhance cycle characteristics and safety or to enable use at a highcharging potential, a part of a lithium composite metal oxide may bereplaced with another element. For example, a part of cobalt, manganese,or nickel may be replaced with at least one or more elements such as Sn,Mg, Fe, Ti, Al, Zr, Cr, V, Ga, Zn, Cu, Bi, Mo, and La, or a part ofoxygen may be replaced with S or F, or the surface of the positiveelectrode may be coated with a compound containing these elements.

For the positive electrode active material, a lithium-containingolivine-type phosphate (LiMPO₄; M is Fe, Mn, Ni, Mg, or Co, etc.) can beused. Specific examples thereof include LiFePO₄, LiMnPO₄, and LiNiPO₄.

One of the positive electrode active materials may be used singly, ortwo or more thereof may be used in combination.

For the purpose of lowering the impedance, a conductive aid may be addedto the positive electrode active material layer comprising the positiveelectrode active material. Specific examples of the conductive aidinclude graphites such as natural graphite and artificial graphite, andcarbon blacks such as acetylene black, Ketjen black, furnace black,channel black, and thermal black. Two or more of these conductive aidsmay be appropriately used in a mixture. The amount of the conductive aidto be added is preferably 1 to 10 parts by mass based on 100 parts bymass of the positive electrode active material.

With regard to the average particle diameter of the positive electrodeactive material, for example, a positive electrode active materialhaving an average particle diameter in the range of 0.1 to 50 μm can beused, preferably a positive electrode active material having an averageparticle diameter in the range of 1 to 30 μm can be used, and morepreferably positive electrode active material having an average particlediameter in the range of 2 to 25 μm can be used, from the viewpoint ofreactivity with the electrolytic solution, rate characteristics, etc.Here, an average particle diameter refers to a particle diameter at 50%of a cumulative value (median diameter: D50) in a particle sizedistribution (volume basis) in a laser diffraction/scattering method.

The binder for positive electrodes is not limited, and examples thereofwhich can be used include polyvinylidene fluorides (PVDF), vinylidenefluoride-hexafluoro propylene copolymers, vinylidenefluoride-tetrafluoroethylene copolymers, styrene-butadiene copolymerrubbers, polytetrafluoroethylenes, polypropylenes, polyethylenes,polyimides, and polyamideimides. Among them, polyvinylidene fluoridesare preferred from the viewpoint of versatility and low cost. The amountof the binder for positive electrodes to be used is, from the viewpointof binding strength and energy density, which are in trade-off relationto the amount of the binder, preferably 2 to 10 parts by mass based on100 parts by mass of the positive electrode active material.

The positive electrode current collector is not limited, and any commonpositive electrode current collector for a typical lithium ion secondarybattery can be used. For a material of the positive electrode currentcollector, aluminum, a stainless steel, or the like can be used.Examples of the shape of the positive electrode current collectorinclude a foil, a sheet, and a mesh. For a suitable positive electrodecurrent collector, an aluminum foil, a lath sheet of a stainless steel,or the like can be used.

In a method for producing the positive electrode, for example, the abovepositive electrode active material, conductive aid, and binder are mixedtogether and a solvent such as N-methylpyrrolidone is added thereto andthe resultant is kneaded to prepare a slurry, and the slurry is appliedonto a current collector by using a doctor blade method, a die coatermethod, or the like, and then dried and pressurized as necessary toproduce the positive electrode.

(Negative Electrode)

For the negative electrode in the lithium ion secondary batteryaccording to an exemplary embodiment, for example, a negative electrodein which a negative electrode active material layer comprising anegative electrode active material and a binder is formed to cover anegative electrode current collector can be used. The binder binds thenegative electrode active material and the current collector, and bindsthe negative electrode active material itself.

Examples of the negative electrode active material include lithiummetal, metals or alloys capable of alloying with lithium, oxides capableof intercalating and deintercalating a lithium ion, and carbonaceousmaterials capable of intercalating and deintercalating a lithium ion.

Examples of the metal or alloy capable of alloying with lithium includeelementary silicon, lithium-silicon alloys, and lithium-tin alloys.

Examples of the oxide capable of intercalating and deintercalating alithium ion include silicon oxides, niobium pentoxide (Nb₂O₅), alithium-titanium composite oxide (Li_(4/3)Ti_(5/3)O₄), and titaniumdioxide (TiO₂).

Examples of the carbonaceous material capable of intercalating anddeintercalating a lithium ion include carbonaceous materials such asgraphite materials (artificial graphite, natural graphite), carbonblacks (acetylene black, furnace black), coke, mesocarbon microbeads,hard carbon, and graphite.

One of the negative electrode active materials may be used singly, ortwo or more thereof may be used in any combination at any ratio.

Among them, carbonaceous materials are preferred in terms ofsatisfactory cycle characteristics and stability and excellentcontinuous charging characteristics.

In terms of capacity, negative electrode active materials comprisingsilicon are preferred. Examples of the negative electrode activematerial comprising silicon include silicon and silicon compounds.Examples of the silicon include elementary silicon. Examples of thesilicon compound include silicon oxides, silicates, and compounds of atransition metal and silicon such as nickel silicide and cobaltsilicide.

Silicon compounds have a function to reduce the swelling and shrinkingof a negative electrode active material itself due to repeatedcharging/discharging, and silicon compounds are more preferred from theviewpoint of charge/discharge cycle characteristics. In addition, somesilicon compounds have a function to ensure the conduction amongsilicons. From such a viewpoint, silicon oxides are preferred for thesilicon compound.

The silicon oxide is not limited, and for example, a silicon oxiderepresented by SiO_(x) (0<×≤2) can be used. The silicon oxide maycomprise Li, and a silicon oxide, for example, represented bySiLi_(y)O_(z) (y>0, 2>z>0) can be used as a silicon oxide comprising Li.The silicon oxide may comprise a trace amount of metal element ornon-metal element. The silicon oxide can contain, for example, one ortwo or more elements selected from nitrogen, boron, and sulfur, forexample, at a content of 0.1 to 5% by mass. A trace amount of metalelement or non-metal element contained can enhance the conductivity ofthe silicon oxide. The silicon oxide may be crystalline or amorphous.

A more suitable negative electrode active material comprises a negativeelectrode active material comprising silicon (preferably, silicon or asilicon oxide) and a negative electrode active material comprising acarbonaceous material capable of intercalating and deintercalating alithium ion. A carbonaceous material can be contained in a negativeelectrode active material comprising silicon (preferably, silicon or asilicon oxide) in a composite state. As is the case with silicon oxides,carbonaceous materials have a function to reduce the swelling andshrinking of a negative electrode active material itself due to repeatedcharging/discharging and ensure the conduction among silicons, being anegative electrode active material. Thus, coexistence of a negativeelectrode active material comprising silicon (preferably, silicon or asilicon oxide) and a carbonaceous material provides more satisfactorycycle characteristics.

For the carbonaceous material, graphite, amorphous carbon, diamond-likecarbon, carbon nanotube, or a composite material thereof can be suitablyused. Graphite, which has high crystallinity, has high conductivity, andis excellent in adhesion to a positive electrode current collectorcontaining metal such as copper and voltage flatness. On the other hand,amorphous carbon, which has low crystallinity, undergoes relativelysmall volume expansion, and thus has a high effect of reducing thevolume expansion of a whole negative electrode and is less likely to bedeteriorated due to unevenness such as grain boundaries and defects. Thecontent of the carbonaceous material in the negative electrode activematerial is preferably 2% by mass or more and 50% by mass or less, andmore preferably 2% by mass or more and 30% by mass or less.

Examples of methods for producing a negative electrode active materialcontaining silicon and a silicon compound include the following method.In the case that a silicon oxide is used for the silicon compound,exemplary methods include a method in which elementary silicon and asilicon oxide are mixed together and calcined at a high temperatureunder reduced pressure. In the case that a compound of a transitionmetal and silicon is used for the silicon compound, exemplary methodsinclude a method in which elementary silicon and a transition metal aremixed together and melt, and a method in which the surface of elementarysilicon is coated with a transition metal by using vapor deposition orthe like.

The above-described production methods can be further combined withformation of a composite with carbon. Examples of such methods include amethod in which a calcined mixture of elementary silicon and a siliconcompound is introduced into an organic compound gas atmosphere at a hightemperature under an oxygen-free atmosphere, and the organic compound iscarbonized to form a coating layer containing carbon, and a method inwhich a calcined mixture of elementary silicon and a silicon compound ismixed with a precursor resin for carbon at a high temperature under anoxygen-free atmosphere, and the precursor resin is carbonized to form acoating layer containing carbon. In this way, a coating layer containingcarbon can be formed on a core containing elementary silicon and asilicon compound (e.g., a silicon oxide). As a result, volume expansiondue to charging/discharging can be suppressed, and a further improvementeffect on cycle characteristics can be achieved.

In the case that a negative electrode active material comprising siliconis used for the negative electrode active material, a compositecomprising silicon, a silicon oxide and a carbonaceous material(hereinafter, also referred to as Si/SiO/C composite) is preferred.Further, the silicon oxide preferably has a totally or partiallyamorphous structure. Silicon oxides having an amorphous structure cansuppress the volume expansion of carbonaceous materials and silicon,which are the other negative electrode active materials. Although themechanism is not clear, the amorphous structure of a silicon oxidepresumably has some effects on film formation in the interface between acarbonaceous material and an electrolytic solution. In addition, theamorphous structure is believed to have relatively few factors due tounevenness such as grain boundaries and defects. X-ray diffractionmeasurement (common XRD measurement) can confirm that a silicon oxidehas a totally or partially amorphous structure. Specifically, in thecase that a silicon oxide does not have an amorphous structure, peaksspecific to the silicon oxide are observed; and in the case that asilicon oxide has a totally or partially amorphous structure, peaksspecific to the silicon oxide are observed as a broad.

Preferably, silicon is totally or partially dispersed in the siliconoxide in the Si/SiO/C composite. At least a part of silicon dispersed inthe silicon oxide can further suppress the volume expansion of a wholenegative electrode, and in addition can suppress the decomposition of anelectrolytic solution. Observation with transmission electron microscopy(common TEM observation) and measurement with energy dispersive X-rayspectroscopy (common EDX measurement) in combination can confirm thatsilicon is totally or partially dispersed in a silicon oxide.Specifically, observation of the cross-section of a sample andmeasurement of oxygen concentration of a part corresponding to silicondispersed in a silicon oxide can confirm that the part is not an oxide.

In the Si/SiO/C composite, for example, the silicon oxide has a totallyor partially amorphous structure, and silicon is totally or partiallydispersed in the silicon oxide. Such an Si/SiO/C composite can beproduced by using a method as disclosed in JP2004-47404A, for example.Specifically, the Si/SiO/C composite can be obtained, for example,through CVD treatment of a silicon oxide under an atmosphere comprisingan organic gas such as methane gas. The Si/SiO/C composite obtained byusing such a method has a form in which the surface of a particlecontaining a silicon oxide comprising silicon is coated with carbon. Inaddition, the silicon is present as nanoclusters in the silicon oxide.

In the Si/SiO/C composite, the fraction of the silicon, the siliconoxide, and the carbonaceous material is not limited. The fraction of thesilicon is preferably 5% by mass or more and 90% by mass or less, andmore preferably 20% by mass or more and 50% by mass or less based on theSi/SiO/C composite. The fraction of the silicon oxide is preferably 5%by mass or more and 90% by mass or less, and more preferably 40% by massor more and 70% by mass or less based on the Si/SiO/C composite. Thefraction of the carbonaceous material is preferably 2% by mass or moreand 50% by mass or less, and more preferably 2% by mass or more and 30%by mass or less based on the Si/SiO/C composite.

The Si/SiO/C composite may be a mixture of elementary silicon, a siliconoxide, and a carbonaceous material, and can be also produced by mixingelementary silicon, a silicon oxide, and a carbonaceous material viamechanical milling. For example, the Si/SiO/C composite can be obtainedby mixing elementary silicon, a silicon oxide, and a carbonaceousmaterial each of which is particulate. For example, the average particlediameter of elementary silicon can be configured to be smaller than theaverage particle diameter of the carbonaceous material and the averageparticle diameter of the silicon oxide. In this configuration,elementary silicon, which undergoes large volume change due tocharging/discharging, has a relatively small particle diameter, and thecarbonaceous material and silicon oxide, which undergo small volumechange, have a relatively large particle diameter, and thus formation ofa dendrite or a fine powder can be effectively prevented. In addition, aparticle of a large particle diameter and a particle of a small particlediameter alternately intercalate and deintercalate a lithium ion duringcharging/discharging, which can prevent the generation of a residualstress and residual strain. The average particle diameter of elementarysilicon can be, for example, 20 μm or smaller, and is preferably 15 μamor smaller. The average particle diameter of the silicon oxide ispreferably ½ or less of the average particle diameter of thecarbonaceous material. The average particle diameter of elementarysilicon is preferably ½ or less of the average particle diameter of thesilicon oxide. It is more preferable that the average particle diameterof the silicon oxide be ½ or less of the average particle diameter ofthe carbonaceous material and the average particle diameter ofelementary silicon be ½ or less of the average particle diameter of thesilicon oxide. If the average particle diameters are controlled withinthe range, an effect of reducing volume expansion can be moreeffectively achieved, and a secondary battery excellent in balance amongenergy density, cycle lifetime, and efficiency can be obtained. Morespecifically, it is preferable that the average particle diameter of thesilicon oxide be ½ or less of the average particle diameter of graphiteand the average particle diameter of elementary silicon be ½ or less ofthe average particle diameter of the silicon oxide. Even morespecifically, the average particle diameter of elementary silicon canbe, for example, 20 μm or smaller, and is preferably 15 μm or smaller.

The average particle diameter of the negative electrode active materialis preferably 1 μm or larger, more preferably 2 μm or larger, and evenmore preferably 5 μm or larger from the viewpoint of suppression of sidereaction during charging/discharging and reduction of lowering ofcharge/discharge efficiency, and preferably 80 μm or smaller, and morepreferably 40 μm or smaller from the viewpoint of input/outputcharacteristics and electrode production (e.g., smoothness of thesurface of an electrode). Here, an average particle diameter refers to aparticle diameter at 50% of a cumulative value (median diameter: D50) ina particle size distribution (volume basis) in a laserdiffraction/scattering method.

For the negative electrode active material, the above-described Si/SiO/Ccomposite the surface of which has been treated with a silane couplingagent or the like may be used.

The negative electrode active material layer preferably comprises theabove negative electrode active material capable of intercalating anddeintercalating a lithium ion as a main component, and specifically, thecontent of the negative electrode active material is preferably 55% bymass or more, and more preferably 65% by mass or more based on the totalof the negative electrode active material layer comprising the negativeelectrode active material and the binder for negative electrodes, andvarious aids, as necessary.

The binder for negative electrodes is not limited, and examples thereofwhich can be used include polyvinylidene fluorides, vinylidenefluoride-hexafluoropropylene copolymers, vinylidenefluoride-tetrafluoroethylene copolymers, styrene-butadiene copolymerrubbers (SBR), polytetrafluoroethylenes, polypropylenes, polyethylenes,polyimides, and polyamideimides. Among them, polyimides,polyamideimides, SBRs, polyacrylic acids (including lithium salts,sodium salts, and potassium salts neutralized with an alkali), andcarboxymethyl cellulose (including lithium salts, sodium salts, andpotassium salts neutralized with an alkali) are preferred because oftheir high binding properties. The amount of the binder for negativeelectrodes to be used is, from the viewpoint of binding strength andenergy density, which are in trade-off relation to the amount of thebinder, preferably 5 to 25 parts by mass based on 100 parts by mass ofthe negative electrode active material.

The negative electrode current collector is not limited, and any commonnegative electrode current collector for a typical lithium ion secondarybattery can be used. For a material of the negative electrode currentcollector, for example, a metal material such as copper, nickel, and SUScan be used. Among them, copper is particularly preferred for ease ofprocessing and cost. It is preferable that the negative electrodecurrent collector have been roughened in advance. Examples of the shapeof the negative electrode include a foil, a sheet, and a mesh. Inaddition, a current collector with holes such as an expanded metal and apunched metal can be used.

In a method for producing the negative electrode, for example, a mixtureof the above-described negative electrode active material and binder,various aids, as necessary, and a solvent is kneaded to prepare aslurry, and the slurry is applied onto a current collector, and thendried and pressurized as necessary to produce the negative electrode, inthe same manner as the above-described production method for thepositive electrode.

(Separator)

The separator is not limited, and a monolayer or multilayer porous filmcontaining a resin material such as a polyolefin includingpolypropylenes and polyethylenes or a nonwoven fabric can be used. Inaddition, a film in which a resin layer of a polyolefin or the like iscoated with a different type of a material or the different type of amaterial is laminated on the resin layer can be used. Examples of suchfilms include a film in which a polyolefin base material is coated witha fluorine compound or an inorganic fine particle, and a film in which apolyolefin base material and an aramid layer are laminated.

The thickness of the separator is preferably 5 to 50 μm, and morepreferably 10 to 40 μm in terms of the energy density and mechanicalstrength of a battery.

(Structure of Lithium Ion Secondary Battery)

The form of the lithium ion secondary battery is not limited, andexamples thereof include a coin battery, a button battery, a cylindricalbattery, a rectangular battery, and a laminate battery.

For example, a laminate battery can be produced as follows: a positiveelectrode, a separator, and a negative electrode are laminatedalternately to form a laminate; a metal terminal called tab is connectedto each electrode; the resultant is contained in a container composed ofa laminate film, as an outer package; and an electrolytic solution isinjected thereinto and the container is sealed.

For the laminate film, any laminate film which is stable in anelectrolytic solution and has sufficient water vapor barrier propertiescan be appropriately selected. For such a laminate film, for example, alaminate film including a polyolefin (e.g., a polypropylene, apolyethylene) coated with an inorganic material such as aluminum,silica, and alumina can be used. In particular, an aluminum laminatefilm including a polyolefin coated with aluminum is preferred from theviewpoint of suppression of volume expansion.

Representative examples of layer configurations for the laminate filminclude a configuration in which a metal thin film layer and aheat-sealable resin layer are laminated. Other examples of layerconfigurations for the laminate film include a configuration in which aresin film (protective layer) containing a polyester such as apolyethylene terephthalate or a polyamide such as a nylon is furtherlaminated on the surface of a metal thin film layer in the side oppositeto a heat-sealable resin layer. In the case that a container including alaminate film containing a laminate including a positive electrode and anegative electrode is sealed, a container is formed so that theheat-sealable resin layers of the laminate film face each other to allowthe heat-sealable resin layers to fuse at a portion for sealing. For themetal thin film layer of the laminate film, for example, a foil of Al,Ti, a Ti alloy, Fe, a stainless steel, a Mg alloy, or the like with athickness of 10 to 100 μm is used. The resin used for the heat-sealableresin layer is not limited and may be any resin capable of beingheat-sealed, and examples thereof include: polypropylenes,polyethylenes, and acid-modified products of them; polyphenylenesulfides; polyesters such as polyethylene terephthalates; polyamides;and ionomer resins in which an ethylene-vinyl acetate copolymer, anethylene-methacrylic acid copolymer, or an ethylene-acrylic acidcopolymer are intermolecularly linked with a metal ion. The thickness ofthe heat-sealable resin layer is preferably 10 to 200 μm, and morepreferably 30 to 100 μm.

FIG. 1 illustrates one example of the structure of the lithium ionsecondary battery according to an exemplary embodiment.

Positive electrode active material layers 1 comprising a positiveelectrode active material are formed on positive electrode currentcollectors 1A to constitute positive electrodes. For the positiveelectrodes are used a single-sided electrode in which a positiveelectrode active material layer 1 is formed on the surface in one sideof a positive electrode current collector 1A, and a double-sidedelectrode in which a positive electrode active material layer 1 isformed on the surface in each side of a positive electrode currentcollector 1A.

Negative electrode active material layers 2 comprising a negativeelectrode active material are formed on negative electrode currentcollectors 2A to constitute negative electrodes. For the negativeelectrodes are used a single-sided electrode in which a negativeelectrode active material layer 2 is formed on the surface in one sideof a negative electrode current collector 2A, and a double-sidedelectrode in which a negative electrode active material layer 2 isformed on the surface in each side of a negative electrode currentcollector 2A.

These positive electrodes and negative electrodes are disposed oppositeto each other via separators 3, as illustrated in FIG. 1, and laminated.The two positive electrode current collectors 1A connect to each otherin one end side, and to the connection a positive electrode tab 1B isconnected. The two negative electrode current collectors 2A connect toeach other in another end side, and to the connection a negativeelectrode tab 2B is connected. The laminate including the positiveelectrodes and the negative electrodes (power generation element) iscontained in a container including an outer package 4, and impregnatedwith an electrolytic solution. The positive electrode tab 1B and thenegative electrode tab 2B protrude out of the outer package 4. Thecontainer is formed in such a way that two rectangle laminate sheets asthe outer package 4 are stacked so as to wrap the power generationelement and the four edge portions are fused for sealing.

EXAMPLE

Hereinafter, the present invention will be described in more detail withreference to Synthesis Examples and Examples, but the present inventionis never limited to these examples.

Synthesis Example 1

Synthesis of difluoroboron complex compound FB1 in which R¹ is 2-thienylgroup, R² is trifluoromethyl group, and R³ is hydrogen atom in generalformula (1)

In 50 mL of dried methylene chloride, 3 g of4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione was dissolved, and 2.34 gof boron trifluoride-diethyl ether complex was added thereto, and theresultant was stirred at room temperature overnight. The solvent isdistilled off under reduced pressure to precipitate a crystal, andhexane was added to the crystal, which was stirred for washing to afford1.896 g (yield: 52%) of the difluoroboron complex FB1 intended.

Synthesis Example 2

Synthesis of difluoroboron complex compound FB2 in which R¹ is phenylgroup, R² is trifluoromethyl group, and R³ is hydrogen atom in generalformula (1)

In 40 mL of dried methylene chloride, 2 g of4,4,4-trifluoro-1-phenyl-1,3-butanedione was dissolved, and 1.602 g ofboron trifluoride-diethyl ether complex was added thereto, and theresultant was stirred at room temperature overnight. The solvent wasdistilled off under reduced pressure to precipitate a crystal, andhexane was added to the crystal, which was stirred for washing. Further,the crystal was dissolved in chloroform, and reprecipitated in hexane toafford 0.91 g (yield: 37%) of the difluoroboron complex FB2 intended.

Synthesis Example 3

Synthesis of difluoroboron complex compound FB3 in which R¹ and R² areeach phenyl group, and R³ is hydrogen atom in general formula (1)

In 20 mL of dried methylene chloride, 1 g of1,3-diphenyl-1,3-propanedione was dissolved, and 0.772 g of borontrifluoride-diethyl ether complex was added thereto, and the resultantwas stirred at room temperature overnight. The solvent was distilled offunder reduced pressure to precipitate a crystal, and hexane was added tothe crystal, which was stirred for washing to afford 0.849 g (yield:70%) of the difluoroboron complex FB3 intended.

Synthesis Example 4

Synthesis of difluoroboron complex compound FB4 in which R¹ and R² areeach 4-fluorophenyl group, and R³ is hydrogen atom in general formula(1)

To 40 mL of dried tetrahydrofuran (THF), 1.76 g of sodium hydride wasadded, and thereto 2 g of 4′-fluoroacetophenone was added dropwise at 0°C. under an argon atmosphere. Subsequently, 2.678 g of methyl4-fluorobenzoate was added thereto, and the resultant was furtherstirred for 30 hours, and then heated to reflux for 2 hours. Thereaction solution was allowed to cool, and then diluted hydrochloricacid was added thereto until the solution became acidic. The organiclayer was extracted with diethyl ether and washed with water. Theorganic layer was dried over magnesium sulfate, and then the solvent wasdistilled off under reduced pressure. The residue was purified in asilica gel column (eluent: chloroform/hexane=5/1 (volume ratio)) toafford 1.0 g of an intermediate A1.

Next, 1.0 g of the intermediate A1 was dissolved in 20 mL of1,2-dimethoxyethane, and 0.82 g of boron trifluoride-diethyl ethercomplex was added thereto, and reacted at 60° C. for 2 hours. Afterallowing to cool, the solvent was distilled off under reduced pressureto precipitate a crystal, and 50 mL of hexane was added to the crystal,which was stirred for washing at 60° C. to afford 0.769 g (yield: 65%)of the difluoroboron complex FB4 intended.

Synthesis Example 5

Synthesis of difluoroboron complex compound FB7 in which R¹ is 2-furanylgroup, R² is trifluoromethyl group, and R³ is hydrogen atom in generalformula (1)

In 30 mL of dried methylene chloride, 5 g of4,4,4-trifluoro-1-(2-furanyl)-1,3-butanedione was dissolved, and 4.2 gof boron trifluoride-diethyl ether complex was added thereto, and theresultant was stirred at room temperature overnight. The solvent wasdistilled off under reduced pressure to precipitate a crystal, andhexane was added to the crystal, which was stirred for washing to afford4.95 g (yield: 80%) of the difluoroboron complex FB7 intended.

Synthesis Example 6

Synthesis of difluoroboron complex compound FB13 in which R¹ is phenylgroup, R² is ethoxy group, and R³ is hydrogen atom in general formula(1)

In 30 mL of dried methylene chloride, 3 g of ethyl benzoylacetate wasdissolved, and 2.7 g of boron trifluoride-diethyl ether complex wasadded thereto, and the resultant was stirred at room temperatureovernight. The solvent was distilled off under reduced pressure toprecipitate a crystal, and hexane is added to the crystal, which wasstirred for washing to afford 2.71 g (yield: 72%) of the difluoroboroncomplex FB13 intended.

(Production Example of Positive Electrode)

LiCo_(1/3)Ni_(1/3)Mn_(1/3)O₂ as a positive electrode active material,carbon black as a conductive aid, and polyvinylidene fluoride as abinder for positive electrodes were weighed at a mass ratio of 94:3:3,and they were mixed with N-methylpyrrolidone to prepare a positiveelectrode slurry. The positive electrode slurry was applied onto onesurface of a positive electrode current collector 1A including analuminum foil with a thickness of 20 μm, and the resultant was dried andfurther pressed to form a positive electrode active material layer 1,and thus a single-sided electrode with a positive electrode activematerial layer formed on one surface of a positive electrode currentcollector was obtained. In the same manner, the positive electrodeactive material layer 1 was formed in each side of the positiveelectrode current collector 1A, and thus a double-sided electrode with apositive electrode active material layer formed on each side of apositive electrode current collector was obtained.

(Production Example of Graphite Negative Electrode)

A graphite powder (94% by mass) as a negative electrode active materialand PVDF (6% by mass) were mixed together, and N-methylpyrrolidone wasadded thereto to prepare a slurry. The slurry was applied onto onesurface of a negative electrode current collector 2A including a copperfoil (thickness: 10 μm), and the resultant was dried to form a negativeelectrode active material layer 2, and thus a single-sided negativeelectrode with a negative electrode active material layer formed on onesurface of a negative electrode current collector was obtained. In thesame manner, the negative electrode active material layer 2 was formedin each side of the negative electrode current collector 2A, and thus adouble-sided electrode with a negative electrode active material layerformed on each side of a negative electrode current collector wasobtained.

(Production Example of Silicon Negative Electrode)

A slurry containing 85% by mass of SiO with an average particle diameterof 15 μm and 15% by mass of polyamic acid was applied onto one surfaceof a negative electrode current collector 2A including a copper foil(thickness: 10 μm), and the resultant was dried to form a negativeelectrode active material layer 2 with a thickness of 46 μm, and thus asingle-sided negative electrode with a negative electrode activematerial layer formed on one side of a negative electrode currentcollector was obtained. In the same manner, the negative electrodeactive material layer 2 was formed in each side of the negativeelectrode current collector 2A, and thus a double-sided electrode with anegative electrode active material layer formed on each side of anegative electrode current collector was obtained. The negativeelectrodes obtained were annealed at 350° C. under a nitrogen atmospherefor 3 hours to cure the binder component.

Example 1

<Preparation of Nonaqueous Electrolytic Solution>

Ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed togetherat a volume ratio (EC/DEC) of 30/70, and LiPF₆ was dissolved therein toa concentration of 1.0 mol/L, and the difluoroboron complex compound FB1synthesized in Synthesis Example 1 was dissolved therein to a content of0.1% by mass to prepare a nonaqueous electrolytic solution.

<Production of Lithium Ion Secondary Battery>

The positive electrode and graphite negative electrode produced in theabove methods were shaped into a predetermined shape, and they werelaminated with a porous film separator 3 sandwiched therebetween, and apositive electrode tab 1B and a negative electrode tab 2B were welded tothe respective electrodes to obtain a power generation element. Thepower generation element was wrapped with an outer package including analuminum laminate films 4, and the three edge portions were heat-sealed,and then the above nonaqueous electrolytic solution was injectedthereinto for impregnation at an appropriate degree of vacuum.Thereafter, the residual one edge portion was heat-sealed under reducedpressure to obtain a pre-activated lithium ion secondary battery havingthe structure illustrated in FIG. 1.

<Step of Activation Treatment>

The pre-activated lithium ion secondary battery produced was subjectedto two cycles repeatedly each of which consists of charging to 4.1 V ata current per gram of the positive electrode active material of 20 mA/g,and discharging to 1.5 V at an identical current per gram of thepositive electrode active material of 20 mA/g.

Example 2

A lithium ion secondary battery was produced in the same manner as inExample 1 except that the silicon negative electrode was used as thenegative electrode in place of the graphite negative electrode.

Example 3

A lithium ion secondary battery was produced in the same manner as inExample 1 except that, in preparation of the nonaqueous electrolyticsolution, 1.0% by mass of the compound FB1 obtained in Synthesis Example1 was added in place of 0.1% by mass of the compound FB1 obtained inSynthesis Example 1.

Example 4

A lithium ion secondary battery was produced in the same manner as inExample 1 except that, in preparation of the nonaqueous electrolyticsolution, 0.1% by mass of the compound FB2 obtained in Synthesis Example2 was added in place of 0.1% by mass of the compound FB1 obtained inSynthesis Example 1.

Example 5

A lithium ion secondary battery was produced in the same manner as inExample 1 except that, in preparation of the nonaqueous electrolyticsolution, 0.1% by mass of the compound FB3 obtained in Synthesis Example3 was added in place of 0.1% by mass of the compound FB1 obtained inSynthesis Example 1.

Example 6

A lithium ion secondary battery was produced in the same manner as inExample 1 except that, in preparation of the nonaqueous electrolyticsolution, 0.1% by mass of the compound FB4 obtained in Synthesis Example4 was added in place of 0.1% by mass of the compound FB1 obtained inSynthesis Example 1.

Example 7

A lithium ion secondary battery was produced in the same manner as inExample 1 except that, in preparation of the nonaqueous electrolyticsolution, 0.1% by mass of the compound FB7 obtained in Synthesis Example5 was added in place of 0.1% by mass of the compound FB1 obtained inSynthesis Example 1.

Example 8

A lithium ion secondary battery was produced in the same manner as inExample 1 except that, in preparation of the nonaqueous electrolyticsolution, 0.1% by mass of the compound FB13 obtained in SynthesisExample 6 was added in place of 0.1% by mass of the compound FB1obtained in Synthesis Example 1.

Example 9

A lithium ion secondary battery was produced in the same manner as inExample 1 except that, in preparation of the nonaqueous electrolyticsolution, 0.05% by mass of the compound FB1 obtained in SynthesisExample 1 and 1.0% by mass of vinylene carbonate were added in place of0.1% by mass of the compound FB1 obtained in Synthesis Example 1.

Example 10

A lithium ion secondary battery was produced in the same manner as inExample 1 except that, in preparation of the nonaqueous electrolyticsolution, 0.05% by mass of the compound FB1 obtained in SynthesisExample 1 and 1.0% by mass of fluoroethylene carbonate were added inplace of 0.1% by mass of the compound FB1 obtained in Synthesis Example1.

Example 11

A lithium ion secondary battery was produced in the same manner as inExample 1 except that, in preparation of the nonaqueous electrolyticsolution, 0.05% by mass of the compound FB1 obtained in SynthesisExample 1 and 0.2% by mass of 1,5,2,4-dioxadithiane-2,2,4,4-tetraoxidewere added in place of 0.1% by mass of the compound FB1 obtained inSynthesis Example 1.

Comparative Example 1

A lithium ion secondary battery was produced in the same manner as inExample 1 except that EC and DEC were mixed together at a volume ratio(EC/DEC) of 30/70, and LiPF₆ as an electrolyte salt was dissolvedtherein to a concentration of 1 mol/L to prepare a solution, and thesolution was used for the electrolytic solution (no additives).

Comparative Example 2

A lithium ion secondary battery was produced in the same manner as inExample 2 except that EC and DEC were mixed together at a volume ratio(EC/DEC) of 30/70, and LiPF₆ as an electrolyte salt was dissolvedtherein to a concentration of 1 mol/L to prepare a solution, and thesolution was used for the electrolytic solution (no additives).

<Evaluation Method for Lithium Ion Secondary Battery>

For the lithium ion secondary batteries produced in Examples 1 to 11 andComparative Examples 1 and 2, cycle characteristics under a hightemperature environment were evaluated.

Specifically, each of the secondary batteries produced was subjected toa test in which charging/discharging was repeated for 50 cycles within avoltage range of 2.5 V to 4.1 V in a thermostat bath kept at 60° C. Andthen, the capacity retention ratio after the cycles was calculated byusing the following formula.Capacity retention rate (%)=(discharge capacity at 50th cycle/dischargecapacity at first cycle)×100<Evaluation Results for Lithium Ion Secondary Batteries>

The compositions of electrolytic solutions, negative electrodematerials, additives, amounts added, evaluation results (capacityretention rates) for Examples and Comparative Examples are summarized inTable 2.

From comparison of Examples 1 to 11 and Comparative Examples 1 and 2, itwas found that a high capacity can be achieved stably by addition of thedifluoroboron complex compound represented by the general formula (1) toan electrolytic solution.

From the results, it was found that the nonaqueous electrolytic solutionaccording to an exemplary embodiment, which contains a specificdifluoroboron complex compound, is effective for enhancement of thecharacteristics (in particular, cycle characteristics under a hightemperature environment) of a lithium ion secondary battery.

TABLE 2 Solvent com- Amount position in added Capacity electrolyticNegative (% by retention solution electrode Additive mass) rate (%)Exam- EC/DEC = graphite compound in 0.1 85 ple 1 30/70 negativeSynthesis electrode Example 1 Exam- EC/DEC = silicon compound in 0.1 65ple 2 30/70 negative Synthesis electrode Example 1 Exam- EC/DEC =graphite compound in 1.0 80 ple 3 30/70 negative Synthesis electrodeExample 1 Exam- EC/DEC = graphite compound in 0.1 83 ple 4 30/70negative Synthesis electrode Example 2 Exam- EC/DEC = graphite compoundin 0.1 80 ple 5 30/70 negative Synthesis electrode Example 3 Exam-EC/DEC = graphite compound in 0.1 83 ple 6 30/70 negative Synthesiselectrode Example 4 Exam- EC/DEC = graphite compound in 0.1 80 ple 730/70 negative Synthesis electrode Example 5 Exam- EC/DEC = graphitecompound in 0.1 79 ple 8 30/70 negative Synthesis electrode Example 6Exam- EC/DEC = graphite compound in 0.05/1.0 78 ple 9 30/70 negativeSynthesis electrode Example 1/ vinylene carbonate Exam- EC/DEC =graphite compound in 0.05/1.0 82 ple 10 30/70 negative Synthesiselectrode Example 1/ fluoroethylene carbonate Exam- EC/DEC = graphitecompound in 0.05/0.2 83 ple 11 30/70 negative Synthesis electrodeExample 1/ 1,5,2,4- dioxadithiane- 2,2,4,4- tetraoxide Com- EC/DEC =graphite none — 20 parative 30/70 negative Exam- electrode ple 1 Com-EC/DEC = silicon none — 16 parative 30/70 negative Exam- electrode ple 2

In the foregoing, the present invention has been described withreference to the exemplary embodiments and the Examples; however, thepresent invention is not limited to the exemplary embodiments and theExamples. Various modifications understandable to those skilled in theart may be made to the constitution and details of the present inventionwithin the scope thereof.

INDUSTRIAL APPLICABILITY

A lithium ion secondary battery using the nonaqueous electrolyticsolution according to an exemplary embodiment exhibits excellentcharacteristics even at a high temperature, and thus can be suitablyutilized for all industrial fields requiring a power supply, andindustrial fields relating to transportation, storage, and supply ofelectrical energy; and specifically, utilized for, for example, a powersupply for mobile devices such as cellular phones, notebook computers,tablet terminals, and portable game machines; a power supply fortravel/transport means such as electrical vehicles, hybrid cars,electric motorcycles, and power-assisted bicycles; an electrical storagesystem for household use; a power supply for backup such as a UPS; andelectrical storage equipment to store power generated throughphotovoltaic power generation, wind power generation, or the like.

The present application claims the right of priority based on JapanesePatent Application No. 2014-152070 filed on Jul. 25, 2014, the entiredisclosure of which is incorporated herein by reference.

REFERENCE SIGNS LIST

-   1: positive electrode active material layer-   1A: positive electrode current collector-   1B: positive electrode tab-   2: negative electrode active material layer-   2A: negative electrode current collector-   2B: negative electrode tab-   3: separator-   4: outer package

The invention claimed is:
 1. A nonaqueous electrolytic solutioncomprising a nonaqueous solvent, an electrolyte salt comprising alithium salt, and a difluoroboron complex compound represented by thefollowing general formula (1):

wherein R¹ and R² each independently represent a substituted orunsubstituted alkyl group having 1-6 carbon atoms, a substituted orunsubstituted aryl group, a substituted or unsubstituted heteroarylgroup, or a substituted or unsubstituted alkoxy group, and R³ representsa hydrogen atom, a substituted or unsubstituted aryl group, or asubstituted or unsubstituted heteroaryl group.
 2. The nonaqueouselectrolytic solution according to claim 1, wherein a content of thedifluoroboron complex compound is in the range of 0.01 to 10% by massbased on a total mass of the nonaqueous electrolytic solution.
 3. Thenonaqueous electrolytic solution according to claim 1, wherein in theformula (1), the substituted or unsubstituted alkyl group having 1-6carbon atoms is an unsubstituted alkyl group having 1-6 carbon atoms oran alkyl group having 1-6 carbon atoms with one or more hydrogen atomsof the alkyl group replaced with a substituent selected from the groupconsisting of a fluorine atom, a cyano group, an ester group having 1-5carbon atoms, an alkoxy group having 1-5 carbon atoms, an aryl group,and a heteroaryl group, the substituted or unsubstituted aryl group isan unsubstituted aryl group or an aryl group with one or more hydrogenatoms of the aryl group replaced with a substituent selected from thegroup consisting of an alkyl group having 1-5 carbon atoms, a fluorineatom, a cyano group, and an alkoxy group having 1-5 carbon atoms, thesubstituted or unsubstituted heteroaryl group is an unsubstitutedheteroaryl group or a heteroaryl group with one or more hydrogen atomsof the heteroaryl group replaced with a substituent selected from thegroup consisting of an alkyl group having 1-5 carbon atoms, a fluorineatom, a cyano group, and an alkoxy group having 1-5 carbon atoms, andthe substituted or unsubstituted alkoxy group is an unsubstituted alkoxygroup having 1-5 carbon atoms or an alkoxy group having 1-5 carbon atomswith one or more hydrogen atoms of the alkoxy group replaced with asubstituent selected from the group consisting of a fluorine atom, acyano group, an aryl group, and a heteroaryl group.
 4. The nonaqueouselectrolytic solution according to claim 1, wherein in the formula (1),R¹ and R² are each independently a group selected from the groupconsisting of a methyl group, a trifluoromethyl group, apentafluoroethyl group, a phenyl group, a 2-thienyl group, a 2-furanylgroup, a 2-fluorophenyl group, a pentafluorophenyl group, a4-fluorophenyl group, a 2,4-difluorophenyl group, a 4-cyanophenyl group,an ethoxy group, and a methoxy group, and R³ is an atom or a groupselected from the group consisting of a hydrogen atom, a phenyl group, a2-thienyl group, a 4-fluorophenyl group, a 2,4-difluorophenyl group, anda pentafluorophenyl group.
 5. The nonaqueous electrolytic solutionaccording to claim 1, further comprising at least one additive compoundselected from the group consisting of vinylene carbonate, fluoroethylenecarbonate, 1,3-propanesultone, maleic anhydride, and1,5,2,4-dioxadithiane-2,2,4,4-tetraoxide.
 6. The nonaqueous electrolyticsolution according to claim 5, wherein a content of the additivecompound is in the range of 0.01 to 10% by mass based on a total mass ofthe nonaqueous electrolytic solution.
 7. The nonaqueous electrolyticsolution according to claim 1, comprising a carbonate as the nonaqueoussolvent.
 8. The nonaqueous electrolytic solution according to claim 1,wherein a concentration of the electrolyte salt is in the range of 0.1to 3 mol/L.
 9. A lithium ion secondary battery comprising: a positiveelectrode comprising a positive electrode active material capable ofintercalating and deintercalating a lithium ion; a negative electrodecomprising a negative electrode active material capable of intercalatingand deintercalating a lithium ion; and the nonaqueous electrolyticsolution according to claim
 1. 10. The lithium ion secondary batteryaccording to claim 9, wherein the negative electrode active materialcomprises at least one selected from the group consisting of elementarysilicon, a silicon oxide, and a carbonaceous material.